Artificial-goosebump-driven microactuation

Microactuators enable precise manipulation at small scales for MEMS, robotics, biomedical devices and electronics. Many existing strategies (magnetic, thermal, photonic, electrical) require intricate active materials with programmed anisotropy, which is difficult to fabricate and reconfigure at microscale. Additionally, capillary-force-induced elastocapillary coalescence during microfabrication can distort or destroy slender structures, and there is a lack of tools to selectively reverse such assemblies. The study addresses these challenges by proposing a light-fuelled microactuation system: a uniaxially aligned liquid crystal elastomer (LCE) “artificial skin” that, when locally heated by a femtosecond laser, creates temporary, localized “artificial goosebumps” to actuate passive, 2PP-printed polymer microstructures. The goals are precise, site-specific, reconfigurable control of microstructure motion (including 2-DOF and full 0–360° rotation), controlled light steering via micro-mirrors, and global/local disassembly of capillary self-assemblies, enabling applications such as information storage.

Magnetically responsive microactuators are popular due to easy fabrication, fast response and large motion ranges. Other actuation modalities (thermal, photonic, electrical) employ hydrogels, LCEs, conducting and piezoelectric materials, offering environmental versatility. Achieving complex motions typically requires spatially programmed anisotropy (e.g., heterogeneous magnetic profiles; molecular alignment in LCEs). However, fine-scale control of anisotropy is challenging, and once programmed, motion modes are often fixed, limiting reconfigurability. Elastocapillary coalescence during drying or development can induce undesirable self-assembly in slender microstructures; mitigation typically uses rigid materials, low aspect ratios, or avoids phase boundaries (e.g., critical point drying), which constrains design flexibility and increases cost/complexity. Although capillary self-assembly can create ordered patterns, its practical use is limited by the lack of post-processing tools for global or local manipulation at small scales. This work builds on prior LCE chemistry (two-step thiol-Michael) and photothermal actuation, adding a mobile nematogenic dopant (5CB) to reduce actuation temperature and localize deformation to overcome thermal diffusion limits in aligned LCEs.

Active skin fabrication: A uniaxially aligned tri-network LCE film (~100 µm thick) was synthesized via a modified two-step thiol-Michael chemistry. The base LCE (crosslinked bi-network of RM257, PETMP, EDDET; catalysts/inhibitors included) was doped with mobile liquid crystal molecules (5CB, typically 25 wt%; also studied 0 and 50 wt%) acting as a plasticizer to lower Tg and TNI and tune deformation. Films were stretched to 150% for uniaxial alignment and UV-crosslinked to fix alignment. Thermal and mechanical properties were characterized (DSC, tensile tests), and anisotropic deformation characterized optically. Passive microstructure fabrication: Passive polymer microstructures (e.g., microhairs, mushroom-like micro-mirrors, micropillar arrays) were directly printed on the LCE surface using two-photon polymerization (2PP) of IP-S photoresist with a femtosecond laser (Nanoscribe; 780 nm, 80 fs, 80 MHz). Structures were designed with aspect ratio <30 for stability. A 20 nm Au layer was sputtered on the LCE surface to enhance photothermal efficiency and mirror reflectivity. Interfacial bonding: Prior to printing, the IP-S resist was incubated on the LCE surface for 30 min to allow slight swelling/interpenetration, then writing initiated 5 µm below the interface to form an interpenetrating network, yielding strong adhesion. A custom detachment test using a load cell measured interfacial strength and energy release rate. Laser actuation and bump generation: Local photothermal heating by scanning a femtosecond laser created temporary microscale artificial goosebumps (local out-of-plane expansion) due to the LCE’s nematic-to-isotropic transition. Laser scanning areas of 1×1 or 5×5 µm² were used; trajectories, speeds and powers were programmed to achieve controlled motion. Arrays of 1 µm-diameter, 2 µm-tall tracking micropillars (10 µm pitch) monitored surface displacement fields. Finite-element simulations: COMSOL Multiphysics (Heat Transfer in Solids and Solid Mechanics) modeled anisotropic thermal conductivity, thermal expansion (shrinkage along director; expansion perpendicular), and resultant surface bumps and temperature fields with Gaussian laser heating. Simulations examined effects of 5CB-induced reductions in thermal conductivity and reproduced elliptical bump shapes. Actuation characterization: Microhair deflections were measured as functions of laser-to-hair spacing S (maximal slope region), hair height H, scanning speed (controls frequency), and power (controls amplitude). 2-DOF motions were realized via linear and circular sweeps enabling 0–360° tip rotations. Micro-mirror designs (reflective plate on four pillars, ~100 µm lateral dimensions) were actuated by selectively lifting pillars via localized bumps to tilt the mirror plane; tilting angles and directions were tracked experimentally and via FE. Applications: (1) Light steering via micro-mirror tilting; (2) Disassembly of capillary-force-induced self-assemblies in slender microstructures by generating disturbance forces (shear/strain) via local bumps to overcome adhesion/cohesion; (3) Information storage using mushroom-like mirror pixels that switch between dark (assembled) and bright (disassembled) states. Uniform bi-assembly pixels were engineered by adding additional spacing Δd between paired units and their neighbors to tune assembly outcomes; large-area patterns, digits, letters, and a QR code were written by selective local disassembly.

- Doping LCE with mobile 5CB molecules lowers Tg and TNI (DSC) and reduces actuation temperature, enabling rapid, reversible anisotropic deformation. Excessive 5CB (>50 wt%) causes yielding/plasticity and reduced actuation amplitude. - Localized artificial goosebumps: A laser scanning area of 1×1 µm² produced a microscale bump with FWHM ~2.8 µm (FE), exhibiting an elliptical profile due to anisotropic deformation. Experimental bump morphology matched simulations. - 5CB doping sharply localizes displacement fields around the laser spot by reducing effective thermal conductivity; displacement outside the scan area falls off steeply for 25–50 wt% doping versus broad, hourglass-like fields without 5CB. - Robust microstructure–substrate adhesion: Interfacial detachment stress reached ~1.93 MPa; measured average interfacial bonding energy was ~24.9 J m⁻² (2–3 orders of magnitude above van der Waals). After >30,000 actuation cycles at 6 Hz, microhairs remained attached with ~85% of initial amplitude retained (~15% decay). - Passive microhair actuation: Hair tips deflect away from the laser and recover upon laser withdrawal. Tip displacement versus spacing S peaks around S ≈ 25 µm (coincident with maximal bump slope). For a given deflection angle, tip displacement scales linearly with hair height H. Higher laser scanning speeds increase actuation frequency; higher power increases deflection amplitude. Programmed trajectories enable 2-DOF motion, including 0–360° rotations via circular sweeps. - Micro-mirror light steering: Local bumps under selected pillars tilt the reflective plane with controllable direction and angle; tilting angle increases with laser power and depends on spot size and mirror scale. Cooperative actuation of multiple pillars enables arbitrary tilt directions (0–360°). - Disassembly of capillary self-assemblies: Laser-induced disturbance forces (shear/strain) combined with elastic restoring forces overcome adhesion/cohesion in assembled slender structures, releasing them to designed states. Large-area disassembly from dark (assembled) to bright (disassembled) states was demonstrated, with mirror centers realigning to pillars. - Information storage: By engineering pixel geometry with additional spacing Δd, assembly outcomes transition from random to tetra-dominant to bi-dominant; 100% bi-assembly yield achieved for Δd > 6 µm, enabling uniform pixels. Selective local disassembly writes patterns (digits 0–9, letters “MPI”, QR code “Hello MPI-IS”). In the QR code example, defective pixels were low: unintended disassembly ~0.16% and unopened pixels ~0.64%.

The work addresses limitations of active-material microactuators at microscale by shifting complexity to a light-responsive LCE “skin” and keeping 3D-printed microstructures passive, thereby simplifying fabrication while retaining rich, reconfigurable motion. Introducing mobile 5CB molecules reduces actuation temperature and suppresses heat diffusion, enabling highly localized, temporary out-of-plane bumps that precisely actuate selected microstructures. This site-specific control allows independent or sequential activation across arrays, achieving two degrees of freedom including full rotations. The approach further resolves a longstanding challenge in slender microfabrication: capillary-force-induced self-assembly. By generating local disturbance forces via bumps, assembled microstructures can be reliably disassembled both locally and globally, enabling functional transitions such as dark-to-bright optical switching. The demonstrated micro-mirror tilting illustrates controllable light steering without integrating complex active elements at each device, and the reversible assembly state enables data encoding. Overall, the findings validate that artificial-goosebump-driven actuation offers precise, programmable, and scalable manipulation of microstructures with applications in micromachines, adaptive optics, and information storage.

This study introduces a light-fuelled, artificial-goosebump microactuation platform that leverages a doped, uniaxially aligned LCE skin to actuate passive, 2PP-printed microstructures with precise, localized, and reconfigurable control. Key contributions include (1) tri-network LCE design with 5CB dopant to lower actuation temperature and localize deformation; (2) strong microstructure–substrate bonding enabling durable, high-frequency actuation; (3) programmable 2-DOF motions up to 360° rotation; (4) functional demonstrations in micro-mirror light steering and in global/local disassembly of capillary self-assemblies; and (5) information storage via selective switching of mirror pixels with low defect rates. Future work could focus on multiplexed independent control over dense arrays, integration with on-chip optics and electronics, further miniaturization, closed-loop optical or thermal feedback for precision, and materials optimization to enhance durability, speed, and energy efficiency.

- Material trade-offs: Excess 5CB (>50 wt%) lowers mechanical robustness and actuation amplitude, risking plastic deformation and reduced performance. - Thermal management: Despite improved localization, anisotropic thermal conduction in aligned LCEs remains a constraint; precise focusing requires careful laser parameter control; broader bumps at suboptimal conditions can unintentionally actuate neighboring structures. - Structural constraints: Printed microstructures maintained aspect ratios <30 for stability; taller/thinner elements may be prone to collapse or unintended assembly. - Durability: Although >30,000 cycles at 6 Hz retained ~85% amplitude, some performance decay (~15%) indicates polymer ageing under repeated heating. - Throughput and scalability: 2PP enables complex 3D forms but is slower than mask-based lithography for mass production; scaling to very large arrays may require hybrid fabrication strategies. - Environmental sensitivity: Capillary assembly/disassembly demonstrations rely on solvent evaporation states and surface conditions; robustness across diverse environments needs further validation.